A sentence is the largest structural unit in terms of grammar organization. There are many ways to classify sentences: using formal terms (declaratives, interrogatives, imperatives, and exclamative), using functional terms (statement, question, command), and simple vs. complex and compound (terms of the number and kind of subjects).
Crystal, David. A Dictionary of Linguistics and Phonetics. The language library. Oxford, UK: Blackwell, 1997.
Matthews, P. H. The Concise Oxford Dictionary of Linguistics. Oxford: Oxford University Press, 1997.